Many thermoplastics have a natural, high gloss finish when injection molded, particularly compositions containing styrenic polymers such as emulsion ABS polymers. For many applications, high gloss is a very desirable characteristic and it may be one of the most important factors in the selection of the material. On the other hand, for many other applications, such as automotive interior applications and information technology equipment, for example computer and other electronic equipment enclosures, there is a trend toward matte or low gloss finishes, principally for aesthetic reasons as well as for the elimination of costly coating and painting steps.
With the recent tendency toward non-coating finish of automotive interior trims such as instrument panels for the purpose of reducing the production costs, there are rapidly growing demands for developing interior parts having excellent appearance such as good low gloss giving improved safety as well as relaxed feeling by reducing light reflection. In addition, the recent tendency to produce interior parts, such as instrument panels, air bag covers and knee bolsters, from the same material creates demand for materials well-balanced in impact resistance and stiffness so as to meet minimum safety requirements.
One technique for obtaining low gloss is to use a mold with a textured and/or grained surface. Texture and/or grain containing molds are even used to mold low gloss materials in order to further accentuate the dull finish. However, even with a low gloss material a textured and/or grained mold may not provide optimum results because the parts do not always have a uniform gloss over a long run. The changes in temperature, internal cavity pressure, and shear rate along the fill path can result in varying degrees of gloss over the surface of the part.
Conventionally, low-cost, general-purpose propylene-based resins have been widely used as materials for automotive interior trims. Polypropylene-based resins naturally exhibit less gloss than some thermoplastic polymers. However, propylene-based resins are deficient in several properties, such as stiffness and scratch resistance. While stiffness and scratch resistance of propylene-based resins can be improved to some extent by the addition of fillers and other additives, the overall balance of properties for propylene-based resins are still unsatisfactory for interior automotive parts.
Other polymers can easily be made to exhibit low gloss by the introduction of one or more additional constituents, or matting agents. An example of such a polymer is high impact polystyrene which is made by mixing polybutadiene with polystyrene. The large domains of rubber within the polystyrene phase result in a low gloss material. However, as with propylene-based resins high impact polystyrene does not demonstrate the balance of properties needed for interior automotive parts. Furthermore, often these matting agents do not assure good replication of texture and/or grain.
Still another inherently low gloss thermoplastic is acrylonitrile, styrene, butadiene (ABS) copolymer made by the bulk, mass, mass-solution, or mass-suspension polymerization process. These processes involve the dissolving of polybutadiene in styrene and acrylonitrile monomers and the subsequent graft reaction between these monomers and the polybutadiene together with acrylonitrile monomer. At the end of the reaction, relatively large particles of rubber are created which may contain some occluded styrene-acrylonitrile copolymer. The large particle size of these rubbery domains is believed to be responsible for the low gloss surface finish of such products. Mass ABS provides an excellent balance of properties including ambient and low temperature impact, stiffness, inherently low gloss, chemical resistance and processability. However, conventional mass ABS resins can demonstrate gloss gradients over the length of an injection molded part, especially a part with texture and/or grain, due to an inherent gloss that is not low enough and to poor replication of the texture and/or grain.
Attempts to improve the ability for ABS to demonstrate very low gloss and a consistent gloss across the surface of a molded part with texture and/or grain have been made by blending ABS with one or more additional resin, such as polycarbonate (PC), polybutylene terephthalate (PBT), polyethylene terephthalte (PET), or polyamides. However, these blends typically result in increased resin cost, higher density, poorer low temperature impact, poorer dimensional stability, water sensitivity, require a compounding step, and/or molding instability at elevated temperatures.
In view of the deficiencies of the materials used for automotive interior parts or information technology equipment applications it would be desirable to have a resin which affords a good balance of properties such as processability, stiffness, impact resistance, and chemical resistance that affords a very low gloss and a consistent low gloss appearance across the surface of a fabricated article having texture and/or grain.